The present invention relates to a call admission control method and a mobile station device for a CDMA mobile communication system that control admission of calls for a frequency bandwidth shared by a plurality of mobile stations which carry out communications with a plurality of radio base stations by a code division multiple access (abbreviated hereafter as CDMA) scheme.
In the currently spread mobile communication system such as portable telephone and automobile telephone, a service is provided by dividing an entire service area into relatively small radio zones called cells. Such a scheme is called a cellular scheme, and a radius of one cell is set to be about 1 to 2 km, for example. By repeatedly reusing the same radio channel geographically, there are advantages such as that a system subscriber capacity can be increased, and that communications can be carried out in good communication quality even by a mobile station with a relatively small transmission power such as a portable telephone terminal because a probability for having a radio base station existing in a short distance from a mobile station becomes high.
In general, in the mobile communication system, there is a limit to the capacity of subscribers that can be accommodated in the system because available radio resources are limited. Consequently, when communication requests in excess of this capacity occur, the so called call loss in which the service providing is refused occurs.
In the conventional FDMA (Frequency Division Multiple Access) or TDMA (Time Division Multiple Access) system in which radio channels are fixedly provided, radio channels to be provided at each radio base station are determined such that there is a sufficiently low probability for having an interference blocking from the identical channel or an interference blocking from neighboring radio channels in the interleave scheme in which radio channels are formed by allowing overlaps in side band power of radio channels. The number of mobile stations that can simultaneously communicate at that radio base station is limited by the number of radio channels provided at that radio base station, and the case of having communication requests in excess of the number of radio channels will result in the call loss. In such a system, the number of channels is allocated to each base station so that there is a drawback in that it is difficult to flexibly deal with uneven distribution or variation in time of traffics.
Also, in the system using the so called dynamic channel allocation in which radio channels are dynamically allocated, a method for selectively allocating radio channels that satisfy a required communication quality at a time of radio channel allocation has been adopted. For example, there are a method in which allocation is permitted when an interference amount is less than or equal to a specified value and a method in which allocation is permitted when a CIR (Carrier to Interference power Ratio) is greater than or equal to a specified value. In this case, the case where the transmission and reception devices provided at each base station are all in use or the case where there is an idle transmission and reception device but no radio channel that satisfies the required communication quality will result in the call loss.
On the other hand, the CDMA scheme is a scheme in which the same radio frequency bandwidth is shared as respective users use different spread codes, and channels are formed by spread codes. In the communication system using this CDMA scheme, all other communications that use the same frequency bandwidth become interferences. Namely, when the same frequency bandwidth is used at all cells, quite many communications at all cells become interference sources so that the communication quality is determined by a total interference amount regardless of which spread codes are used for communications by respective users.
Consequently, there is a problem that the communication quality cannot be guaranteed in the CDMA system by the method which selects a radio channel that satisfies a channel reuse distance or a required communication quality, as has been carried out in the conventional FDMA or TDMA system.
For this reason, there is a need to suppress the number of interference sources itself in order to guarantee the communication quality in the CDMA system. As a technique for suppressing the interference amount, various techniques such as the sectorization, voice activation, etc., are known, but in order to suppress the interference amount less than or equal to a reference value when their conditions are given, it is necessary to limit the number of users who make connections simultaneously.
From these viewpoints, Japanese Patent Application No. 7-759 (1995) discloses a call admission control method for the purpose of guaranteeing the communication quality in the CDMA scheme. Also, Japanese Patent Application No. 7-326056 (1995) discloses a method for judging call admission permission/refusal by measuring an uplink interference amount at a radio base station and comparing the measurement result and a predetermined threshold, and a method for setting up the threshold.
Japanese Patent Application No. 7-759 (1995) is characterized in that, upon newly admitting a call, when the communication qualities of a mobile station corresponding to that call and mobile stations already connected to that radio base station are estimated by referring to the interference amount at a time of newly admitting a call that is estimated according to the past data, if it is estimated that there exists a mobile station that cannot satisfy the prescribed communication quality, the admission of this new call is refused.
Also, Japanese Patent Application No. 7-326056 (1995) obtains the maximum offered traffic in a given frequency bandwidth in order to guarantee the required communication quality and satisfy the prescribed call loss rate, from the first relationship between the offered traffic and the threshold of the interference amount for the call admission in order to guarantee the required communication quality which is obtained according to the communication quality degradation rate and the second relationship between the offered traffic and the threshold of the interference amount for the call admission in order to satisfy the required call loss rate which is obtained according to the call loss rate, and controls admission of a new call by determining the threshold of the interference amount for the call admission in that frequency bandwidth in order to satisfy the required call loss rate with respect to the offered traffic within a range of this maximum offered traffic according to said second relationship.
However, these call admission control methods make the judgement of admission permission/refusal at a radio base station, so that they have a serious, practically unfavorable drawback that an increase in the control signal amount between the radio base station and the mobile station is caused especially when the traffic is large at that radio base station. This is generally due to the occurrence of a phenomenon called re-calling in which users who were not connected as it was judged that admission is refused will repeat the call origination operation successively. Usually, in order to originate a call, an access is made from a mobile station to a radio base station by using a channel that is commonly used by respective users. When many users for which it was judged that admission is refused repeat the call origination operation successively, it causes a vicious circle in which the traffic of this common channel increases abruptly so that a probability for being admitted becomes even lower. In the CDMA, when the same radio frequency bandwidth is used for this common channel and communication channels, there has been a problem that it does not stop to be just a problem of lowering of efficiency in the common channel and it also causes the degradation of the communication quality as these control signals become large interferences even with respect to mobile stations in communications.
Moreover, in the methods disclosed in Japanese Patent Application No. 7-326056 (1995) and Japanese Patent Application No. 7-759 (1995), the judgement of call admission permission/refusal is made closedly at one radio base station, so that there has been a serious, practically unfavorable problem that a mobile station that is admitted to that radio base station gives a large interference to the surrounding radio base stations and as a result it is impossible to suppress the possibility for degrading the communication qualities of mobile stations which are carrying out communications by making connections to the surrounding base stations.
The object of the present invention is to provide a call admission control method and a mobile station device for a CDMA mobile communication system which is capable of not damaging a communication quality of a mobile station in communication even under a situation of large traffic, suppressing a possibility for degrading a communication qualities of mobile stations in communication with surrounding base stations sufficiently low, and improving efficiency without increasing the control signal traffic between the base station and the mobile station, between the base station and surrounding base stations, and between the base station and an upper level control station.
The present invention is a call admission control method for a CDMA mobile communication that controls admission of calls for a frequency bandwidth shared by a plurality of mobile stations which carry out communications with radio base stations by a code division multiple access, in which a radio base station broadcasts a call admission information indicating a state of own station regarding admission of a new call using a broadcast channel, and a mobile station acquires said call admission information which is broadcast from a connection target radio base station through the broadcast channel, and judges admission permission/refusal of said mobile station according to this acquired call admission information.
In this way, it is possible to reduce the control signal traffic between the base station and the mobile station, while preventing degradation of communication qualities of users who are already connected with the connection target base station and in communication.
Also, in another aspect of the present invention, the mobile station acquires the call admission information at respective radio base stations which are broadcast from surrounding radio base stations of the connection target radio base station through broadcast channels transmitted by respective radio base stations, and uses these acquired call admission information in judging admission permission/refusal of said mobile station in addition.
In this way, it is possible to further reduce the control signal traffic between the base station and the mobile station, while preventing degradation of communication qualities of users who are already connected with the connection target base station and in communication.
Also, in another aspect of the present invention, the call admission information indicates either permission or refusal regarding admission of a new call at own radio base station.
Also, in another aspect of the present invention, the radio base station measures an uplink interference amount at own base station, and the call admission information is formed at least by said measured uplink interference amount and a prescribed threshold.
Also, in another aspect of the present invention, the radio base station measures an uplink interference amount at own base station and calculates a remaining capacity at own base station according to this measured uplink interference amount, and the call admission information indicates at least this calculated remaining capacity.
Also, in another aspect of the present invention, the radio base station measures an uplink interference amount at own base station, and the call admission information is formed at least by said measured uplink interference amount at own base station, and the mobile station calculates a required receiving level at the radio base station that is expected in a case where it is admitted to the connection target radio base station, according to the interference amount acquired from the broadcast channel, and judges that admission of said mobile station is refused in a case where it is judged that own station cannot satisfy a required communication quality, according to said acquired uplink interference amount and said calculated required receiving level.
In this way, it is possible to reduce the control signal traffic between the base station and the mobile station, while preventing degradation of communication qualities of users who are already connected with the connection target base station and in communication.
Also, in another aspect of the present invention, the mobile station estimates an uplink interference amount after said mobile station is admitted, from the uplink interference amount at the base station and the required receiving level that is expected in a case where said mobile station is admitted, and judges that admission of said mobile station is refused in a case where this estimated uplink interference amount after admission exceeds a prescribed threshold.
In this way, it is possible to reduce the control signal traffic between the base station and the mobile station, while preventing degradation of communication qualities of users who are already connected with the connection target base station and in communication.
Also, in another aspect of the present invention, the radio base station measures an uplink interference amount at own base station, and the call admission information is formed at least by said measured uplink interference amount at own base station, and the mobile station calculates an interference amount to be given to surrounding base stations that is expected in a case where said mobile station is admitted to the connection target radio base station, according to a required receiving level at the connection target radio base station that is expected in a case where it is admitted to the connection target radio base station, estimates an uplink interference amount at surrounding base stations after said mobile station is admitted to the connection target radio base station, from said acquired uplink interference amount at surrounding base stations, and said calculated interference amount to be given to surrounding base stations, and judges that admission of said mobile station is refused when this estimated uplink interference amount at surrounding base stations exceeds a prescribed threshold.
In this way, it is possible to reduce the control signal traffic between the base station and the mobile station, while preventing degradation of communication qualities of users who are already connected with the connection target base station and in communication.
Also, in another aspect of the present invention, the the radio base station measures a transmission power of own base station, and the call admission information is formed at least by said measured transmission power value of the radio base station and a prescribed threshold.
Also, in another aspect of the present invention, the radio base station measures a transmission power of own base station and calculates a remaining capacity at own base station according to said measured transmission power value of the radio base station, and the call admission information indicates at least this calculated remaining capacity.
Also, in another aspect of the present invention, the call admission information is formed at least by a number of spread codes currently used within own radio base station or a number of hardware resources currently used within own radio base station, and a threshold corresponding to them.
Also, in another aspect of the present invention, the radio base station calculates a remaining capacity at own base station according to a number of spread codes currently used within own radio base station or a number of hardware resources currently used within own radio base station, and the call admission information indicates at least this calculated remaining capacity.
Also, the present invention is a mobile station device in a CDMA mobile communication system for controlling admission of calls for a frequency bandwidth shared by a plurality of mobile stations which carry out communications with radio base stations by a code division multiple access, which has acquisition means for acquiring a call admission information indicating a state regarding admission of a new call at a radio base station which is broadcast from a connection target radio base station through a broadcast channel, judging means for judging admission permission/refusal of said mobile station according to this acquired call admission information, and call processing terminating means for terminating a call processing operation at said mobile station in a case where it is judged that admission is refused as a result of this judging.
In this way, it is possible to reduce the control signal traffic between the base station and the mobile station, while preventing degradation of communication qualities of users who are already connected with the connection target base station and in communication.
Also, in another aspect of the present invention, the acquisition means also acquires the call admission information at respective radio base stations which are broadcast from surrounding base stations of the connection target radio base stations through broadcast channels transmitted by respective base stations.
Also, in another aspect of the present invention, the acquisition means acquires an uplink interference amount from the call admission information which is broadcast from the connection target radio base station, and the judging means calculates a required receiving level at the radio base station that is expected in a case where it is admitted to the connection target radio base station, according to the interference amount acquired from the broadcast channel, and judges that admission of said mobile station is refused in a case where it is judged that own station cannot satisfy a required communication quality, according to said acquired uplink interference amount and said calculated required receiving level.
In this way, it is possible to reduce the control signal traffic between the base station and the mobile station, while preventing degradation of communication qualities of users who are already connected with the connection target base station and in communication.
Also, in another aspect of the present invention, the judging unit estimates an uplink interference amount after said mobile station is admitted, from the uplink interference amount at the base station and the required receiving level that is expected in a case where the mobile station is admitted, and judges that admission of said mobile station is refused in a case where this estimated uplink interference amount after admission exceeds a prescribed threshold.
Also, in another aspect of the present invention, the acquisition means acquires an uplink interference amount at the base station from the call admission information that is broadcast through the broadcast channel, and the judging unit calculates an interference amount to be given to surrounding base stations that is expected in a case where said mobile station is admitted to the connection target radio base station, according to a required receiving level at the connection target radio base station that is expected in a case where it is admitted to the connection target radio base station, estimates an uplink interference amount at surrounding base stations after said mobile station is admitted to the connection target radio base station, from said acquired uplink interference amount at surrounding base stations, and said calculated interference amount to be given to surrounding base stations, and judges that admission of said mobile station is refused when this estimated uplink interference amount at surrounding base stations exceeds a prescribed threshold.
In this way, it is possible to reduce the control signal traffic between the base station and the mobile station, while preventing degradation of communication qualities of users who are already connected with surrounding base stations of the connection target base station and in communication.